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The more restrictive airspace regulations force drone manufacturers to take into account the noise emitted by the drone during the design phase, along with its aerodynamic performance to increase the flight time. A Non-Linear Vortex Lattice Method (NVLM), coupled with the Farassat Formulation-1A of the Ffowcs-Williams and Hawkings acoustic analogy is used to evaluate the aerodynamic and aeroacoustic performance of MAV rotors. Pymoo, a Python-based optimization framework, is employed to modify the geometry, evaluate its performance and extract the set of Pareto optimal solutions. The two objectives are the aerodynamic Figure-of-Merit and the Sound Pressure Level of the Blade Passing Frequency tone for a microphone located at a far-field distance of 1.62 m and 30 ∘ below the rotor plane. The approach proposed in this paper takes into account up to ten different parameters, ranging from the twist and chord distributions, to the rake and skew angles. [ABSTRACT FROM AUTHOR]

Micro air vehicles with transitioning flight capabilities, or simply hybrid micro air vehicles, combine the beneficial features of fixed-wing configurations, in terms of endurance, with vertical take-off and landing capabilities of rotorcrafts to perform five different flight phases during typical missions, such as vertical takeoff, transitioning flight, forward flight, hovering and vertical landing. This promising micro air vehicle class has a wider flight envelope than conventional micro air vehicles, which implies new challenges for both control community and aerodynamic designers. One of the major challenges of hybrid micro air vehicles is the fast variation of aerodynamic forces and moments during the transition flight phase which is difficult to model accurately. To overcome this problem, we propose a flight control architecture that estimates and counteracts in real-time these fast dynamics with an intelligent feedback controller. The proposed flight controller is designed to stabilize the hybrid micro air vehicle attitude as well as its velocity and position during all flight phases. By using model-free control algorithms, the proposed flight control architecture bypasses the need for a precise hybrid micro air vehicle model that is costly and time consuming to obtain. A comprehensive set of flight simulations covering the entire flight envelope of tailsitter micro air vehicles is presented. Finally, real-world flight tests were conducted to compare the model-free control performance to that of the Incremental Nonlinear Dynamic Inversion controller, which has been applied to a variety of aircraft providing effective flight performances. [ABSTRACT FROM AUTHOR]

The demand in micro-air vehicles is increasing as well as their potential missions. Either for discretion in military operations or noise pollution in civilian use, noise reduction of micro-air vehicles is a goal to achieve. Aeroacoustic research has long been focusing on full scale rotorcrafts. At micro-air vehicle scales however, the hierarchization of the numerous sources of noise is not straightforward, as a consequence of the relatively low Reynolds number that ranges typically from 5000 to 100,000 and low Mach number of approximately 0.1. This knowledge, however, is crucial for aeroacoustic optimization and blade noise reduction in drones. This contribution briefly describes a low-cost, numerical methodology to achieve noise reduction by optimization of micro-air vehicle rotor blade geometry. Acoustic power measurements show a reduction of 8 dB(A). The innovative rotor blade geometry allowing this noise reduction is then analysed in detail, both experimentally and numerically with large eddy simulation using lattice Boltzmann method. Turbulence interaction noise is shown to be a major source of noise in this configuration of low Reynolds number rotor in hover, as a result of small scale turbulence and high frequency unsteady aeroadynamics impinging the blades at the leading edge. [ABSTRACT FROM AUTHOR]

Over the last 10 years, the use of micro air vehicles has rapidly covered a broad range of civilian and military applications. While most missions require optimizing the endurance, a growing number of applications also require acoustic covertness. For rotorcraft micro air vehicles, combining endurance and covertness heavily relies on the capability to design new propulsion systems. The present paper aims at describing a complete methodology for designing quiet and efficient micro air vehicle rotors, ranging from preliminary aerodynamic prediction to aeroacoustic optimization to experimental validation. The present approach is suitable for engineering purposes and can be applied to any multirotor micro air vehicle. A fast-response and reliable aerodynamic design method based on the blade-element momentum theory has been used and coupled with an extended acoustic model based on the Ffowcs Williams and Hawkings equation as well as analytical formulations for broadband noise. The aerodynamic and acoustic solvers have been coupled within an optimization tool. Key design parameters include the number of blades, twist and chord distribution along the blade, as well as the choice of an optimal airfoil. An experimental test bench suitable for non-anechoic environment has been developed in order to assess the benefit of the new rotor designs. Optimal rotors can maintain high aerodynamic efficiency and low acoustic signature with noise reductions in the order of 10 dB(A). [ABSTRACT FROM AUTHOR]

The demand for micro-air vehicles is increasing as well as their potential missions. Whether for discretion in military operations or noise pollution in civilian use, the improvement of aerodynamic and acoustic performance of micro-air vehicles propeller is a goal to achieve. Micro- and nano-air vehicles operate at Reynolds numbers ranging from 103 to 105. In these conditions, the aerodynamic performance of conventional fixed and rotary wings concepts drastically decreases due to the increased importance of flow viscous forces that tend to increase drag and promote flow separation, which leads to reduced efficiency and reduced maximum achievable lift. Reduced efficiency and lift result in low endurance and limited payloads. The numerical simulation is a potential solution to better understand such low Reynolds number flows and to increase the micro-air vehicles’ performance. In this paper, it is proposed to review some challenges related to micro-air vehicles by using a Lattice-Boltzmann method. The method is first briefly presented, to point out its strengths and weaknesses. Lattice-Boltzmann method is then applied to three different applications: a DNS of a single blade rotor, a large eddy simulation of a rotor operating in-ground effect and a large eddy simulation of a rotor optimised for acoustic performance. A comparison with reference data (Reynolds Averaged Navier-Stokes, DNS or experimental data) is systematically done to assess the accuracy of lattice-Boltzmann method-based predictions. The analysis of results demonstrates that lattice-Boltzmann method has a good potential to predict the mean aerodynamic performance (torque and thrust) if the grid resolution is chosen adequately (which is not always possible due to limited computational resources). A study of the turbulent flow is conducted for each application in order to highlight some of the physical flow phenomena that take place in such rotors. Different designs are also investigated, showing that potential improvements are still possible in terms of aerodynamic and aero-acoustic performance of low-Reynolds rotors. [ABSTRACT FROM AUTHOR]

One of the major challenges of Mini-Unmanned Aerial Vehicle flight is the unsteady interaction with turbulent environment while flying in lower levels of atmospheric boundary layer. Following inspiration from nature we expose a new system for angle of attack estimation based on pressure measurements on the wing. Such an equipment can be used for real-time estimation of the angle of attack during flight or even further building of wind velocity vector with additional equipment. Those information can find purpose in control and stabilization of the aircraft due to inequalities seen by the wing or even for various soaring strategies that rely on active control for energy extraction. In that purpose, flying wing aircraft has been used with totally four span-wise locations for local angle of attack estimation. In-flight angle of attack estimation from differential pressure measurements on the wing has been compared with magnetic sensor with wind vane. The results have shown that pressure ports give more reliable estimation of angle of attack when compared to values given by wind vane attached to a specially designed air-boom. Difference in local angle of attack at four span-wise locations has confirmed spatial variation of turbulence in low altitude flight. Moreover, theoretical law of energy dissipation for wind components described by Kaimal spectrum has shown acceptable match with estimated ones. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of airfoil at ultra-low Reynolds number has a great impact on the propulsive performance of nano rotor. Therefore, the optimization of airfoil is necessary before the design of nano rotor. Nano rotor blade airfoil optimization is a multi-objective problem since the airfoil suffers a wide range of Reynolds number which increases the difficulty of optimization. In this paper, the airfoil of nano rotor was optimized based on the controlled elitist Non-dominated Sorting Genetic Algorithm II (NSGA-II) coupling with the parameterization method of Class function/Shape function Transformation technique (CST) and the multi-objectives function processing method of statistical definition of stability. An airfoil was achieved with the thickness of 2% and the maximum camber of 5.6% at 2/3 of chord. Airfoil optimized exhibits a good aerodynamic performance at ultra-low Reynolds number according to the computational results. And comparisons were carried out between the performance of the rotor designed with airfoil optimized and that of the rotor designed with AG38 airfoil, which showed that the airfoil optimized was suitable for rotor design. [ABSTRACT FROM AUTHOR]

A passive twist control is considered as an adaptive way to maximize the overall efficiency of a proprotor developed for convertible Micro Air Vehicles. The varied operation conditions suggest different twist distributions for hover and forward flight with the constraint of identical planform. In this work, adaption of the proprotor geometry is achieved by centrifugal force induced twist. Classical Lamination Theory is used to predict structural loads, while Blade Element Momentum Theory is employed to understand the aerodynamic benefits of adaptive proprotor as applied on Micro Air Vehicles. Tip mass is proposed to increase the stability and generate negative torsion for rotating blade. Validation of the procedure is based on measurements of blade deformation, performed by laser displacement sensors. While negative torsion is found in both rotor and propeller modes, level of deformation is still below what is required for optimum dual operation. [ABSTRACT FROM AUTHOR]

An experimental investigation of the interaction of a propeller-wing configuration for a tilt body MAV VTOL was performed in the low speed wind tunnel. This study's primary objective is to present the effect of the interactions between a low aspect ratio wing and propeller for a range of incidence in transition between horizontal and vertical flight. During the transition from horizontal flight to vertical flight or vice versa, the flow patterns seen by the wing are the result of the combination between the free-stream and the propeller flow. This was reflected in the change of the aerodynamic forces and moments of the wing. The model is a tractor configuration propeller and with a wing of aspect ratio equal to one, the airfoil of the wing is a NACA 0012. All tests were conducted at low speeds in a range from 2 to 8 m/s. In order to simulate the transition flight of a tilt-body MAV VTOL a range of incidence from -10 to 90 degrees was used. The results show that the flow of the propeller certainly improves the aerodynamic characteristics of the wing, increasing the lift and delaying stall with respect to the flight path of the MAV. [ABSTRACT FROM AUTHOR]

This study presents the effort given for the first prototype of a Long Endurance Mini UAV concept called Eternity. A multi-disciplinary conceptual aircraft design program called CDSGN is developed and used for the design of the Eternity. Unlike the traditional design methods that uses statistical data from the previous well-flown aircrafts, CDSGN analyses numerous aircraft candidates and simulates each candidate for the given mission definition and outputs the corresponding performance. The unique property of the presented design methodology comes from a computationally fast and physically accurate modelling of the aerodynamic characteristics of each candidate by using a modified version of a vortex lattice program called AVL from Mark Drela. Two types of configurations have been analysed for the Eternity design, conventional and flying-wing. Awide envelope of variable design parameters used for both configurations such as wing surface area, cruise speed, battery capacity, different airfoils, etc.. Integration of solar cells, and the management of solar energy is also considered for every candidate. Only the wing span size has been limited to one meter. Additionally, the on-board avionics and payload weights and sizes are fixed for every candidate as they are independent of the design. Analyses by CDSGN concluded the dominance of the conventional configuration for the given long endurance mission performance both on solar and non-solar conditions. Optimum wing surface area and the on-board battery energy found interactively by a post-filtering program developed in-house. A custom airfoil family, transitioning along the span, have been designed specifically for the corresponding local Reynolds number for specific spanwise locations. A wind-tunnel campaign is performed with a full-scale model and first flight tests have been performed in order to show the feasibility of long endurance flights. [ABSTRACT FROM AUTHOR]

The static performance of nano-scale rotor and the mechanical efficiency of micro rotors are important factors to evaluate rotary-wing NAVs. In this paper, two pairs of nano-scale rotors, which were formed with the same airfoil section for each pair, were compared to find the influence of chord distribution and twist distribution to rotor performance. A test bench was designed with highly sensitive mechanism systems in order to be able to measure the thrust and torque of nano-rotors accurately. The static performance of nano-scale rotors was evaluated experimentally with the test bench at ultra-low Re. And computations based on 3D unsteady incompressible Navier-Stokes solver with artificial compressibility were carried out as well to obtain the detailed flow field of nano rotor. It was found that the rotor figure of merit degraded a lot with size reduction. And the rotor solidity determined by chord distribution has a positive effect on the rotor thrust coefficient while pitch angle distribution influences the power coefficient. Flow field analysis indicated that the state of leading edge vortex might be the inherent reason of the difference of performance between rotors. The mechanical efficiency of several small motors were also compared which showed that the performance of small rotors decline with the reduction of size. [ABSTRACT FROM AUTHOR]

Wind tunnel testing was performed on a VTOL aircraft in order to characterize longitudinal flight behavior during an equilibrium transition between vertical and horizontal flight modes. Trim values for airspeed, pitch, motor speed and elevator position were determined. Data was collected by independently varying the trim parameters, and stability and control derivatives were identified as functions of the trim pitch angle. A linear fractional representation model was then proposed, along with several methods to improve longitudinal control of the aircraft. [ABSTRACT FROM AUTHOR]

Multi-mission Micro Air Vehicles (MAVs) are now becoming more challenging and attractive. Therefore, high speed, low speed and hovering capacities should be studied and performed. The aerodynamics of these new MAVs are very complex due to several problems including very low aspect ratio wings, very low Reynolds number (Re) flows, very high angle of attack flight, and very high interaction between the propeller and the airframe. These complex aerodynamics can not be yet exactly determined by numerical tools. Therefore, research and design studies of multi-mission MAVs should be conducted by wind tunnel testing. This paper summarizes the developments of low speed low Reynolds testing facilities for multi mission MAV research at ISAE-SUPAERO. The experimental facilities have been modified and developed in order to obtain good experimental results at low speed and low Re numbers which include: a) improvement in low speed stability of the wind tunnel, b) development of a new high precision balance for wing-propulsive interaction test, and c) longitudinal gust generator system for flexible wing studies. The performance and advantage of the facilities has been proved by research and successful design of a low speed fixed wing MAV (the TYTO) and VTOL MAV (the MiniVertiGo and the MAVion). [ABSTRACT FROM AUTHOR]

The article presents an experimental analysis of ground effect on a fixed-wing vertical take-off and landing (VTOL) Micro Air Vehicles (MAVs), called MAVion. It says that the VTOL MAV was developed by the University of Arizona and Institut Supérieur de l'A éronautique et de l'Espace (ISAE) in 2007, which is used in reconnaissance missions. The hover flight performances were assessed in terms of aerodynamic efficiency and propulsion. Ground effects were examined in consideration of predicting the vehicle behaviour in take-off and landing phases with the use of an acquired 5-component Micro-Sting Balance (MicroB). The MicroB balance's validation test shows that the balance has sufficient accuracy and the ground has effects on MaVion's aerodynamic characteristics.

The article presents a study on test benches which are design to measure the thrust and torque of nano-rotors to evaluate the rotary wing nano air vehicles (NAVs). The experiment uses data processing, and computational method to MICRO brushless out-runner motor and MCF32254 carbon rotor from MicroInvent to determine the hovering performance measurement of Nano-rotors. Experimental results show that the thrust coefficient differs from diverse test benches while the power coefficient differ by a factor of about two, torque sensor bench has the smallest errors, and test bench respond to the variations of loads quickly. It also presents several charts on the hovering performance of thrust and torque of nano-rotors.

A conceptual design and performance analysis method (Long Endurance Conceptual Design Program) for long-endurance mini-micro UAVs is presented. Recent long endurance oriented results and achievements are looked through for possible usage for mini-micro scale. A real mission is also explained, whose objective is to accomplish a 200 km straight line flight autonomously with the smallest electric platform possible. Design phases of the platform by using the presented method, flight tests and comparison of the results are included. On the following section a design study for long-endurance MAVs using a hybrid energy system combining solar energy and Lithium batteries and the effect of size and cruise speed are investigated. We demonstrate that under a certain size, the use of solar energy becomes not useful at all. We conclude with the study of a candidate design for EMAV09 Endurance Mission in the light of the rules and scoring of the mission. [ABSTRACT FROM AUTHOR]